1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more specifically to programmable logic devices.
2. Description of the Prior Art
Programmable logic devices having and-or arrays are currently in wide use in the electronics industry. These devices provide sum-of-products logic functions. The sum-of-products functions are typically combined within output logic macrocells for performing additional logic functions.
The and-or array is programmed at a program time for the device. The programing defines selected logic functions which are to be performed by the device. Storage cells within the programmable logic device retain the programing information, so that the selected function is retained in the device. Typically, in MOS devices, nonvolatile memory elements such as EPROM or EEPROM cells are used to retain the programing information.
Use of EPROM and EEPROM elements for programing the programmable logic devices has several disadvantages. The programing time for the nonvolatile bits is relatively long, leading to long program times if a large number of complex devices must be programmed. Reprogramming an already programmed device is also a somewhat complex procedure. More complex integrated circuit processing is required to fabricate the EPROM or EEPROM devices than is otherwise required for the remaining elements of the device.
Presently available designs draw a fairly high current during operation and during quiescent conditions. This occurs in part because of the operating requirements of the EPROM or EEPROM cells. These designs use sense amplifiers to detect small voltage swings and amplify them to rail-to-rail voltage changes. The sense amps draw a fairly large amount of current whether or not they are being used.
It would be desirable to provide a cell suitable for use with and-or array programmable logic devices which is fast and which draws a small amount of current. It would further be desirable for such a device to be easily fabricated using available technology.